Bath room over heated.

[Tooth Ferry]

2014 Montana High Country 305 RL. We don't use the furnace often as we can usually get by with the fireplace and a space heater when on electric hook-up but we just spent a week in GSMNP with no hook-ups and temps in lower 30s at night. There is a 4 or 5" opening in the bath floor behind the toilet that does not have a duct attached to it but must be right in the furnace heat chamber area as it blows very hot air and probably blows out at least 75% if not more of the total furnace output. It is very uncomfortable to be in the bathroom over a minute or two with the door shut if the furnace is on. I tried placing a book over the vent and in a couple of minutes the furnace shut off and the book felt like it was close to catching on fire. Has anyone modified any thing in this area to get more of this super hot air distributed to the living room or bed room? I don't know if there is a sensor that shut the furnace down when I tried covering this vent,don't want to cause a fire. Is this supposed to be attached to a duct or vented into basement?